Organic electroluminescent field effect transistors, also known as OLETs (Organic Light-Emitting Transistors), are a relatively recent type of devices that combine the switching mechanism of a thin-film transistor and an electroluminescent device. While charge transport occurs perpendicular to the organic layers in OLEDs (Organic Light-Emitting Diodes), the majority of the current flows horizontally through the semiconducting layers in OLETs. As a result, light in OLETs can be emitted as a stripe along the emissive layer, rather than uniformly through the electrode areas as in conventional OLEDs. The planar transport geometry of OLETs helps suppress deleterious photon losses and exciton quenching mechanisms inherent in the OLED architecture. Accordingly, the same organic electroluminescent light-emitting material has been shown to achieve much higher external quantum efficiency (EQE) and luminosity in an OLET than in an equivalent OLED.
European Patent No. EP 1609195 describes an OLET which has an ambipolar channel that can be realized by one layer or several coplanar layers of organic semiconductor(s). Further details about the functional characteristics of such OLETs and their advantages over OLEDs can be found in Capelli et al., “Organic light-emitting transistors with an efficiency that outperforms the equivalent light-emitting diodes,” Nature Materials, vol. 9, pages 496-503 (2010).
So far, various studies and characterizations of OLETs have shown that enhanced luminosity can be obtained at bias conditions where the efficiency of charge current conversion into light emission tends to be very low (in the order of 1×10−1%). Conversely, device efficiency usually can be maximized by modifying the bias conditions but with detrimental effects on luminosity. Such limitations significantly reduce the usefulness of current state-of-the-art OLETs in applications when both high brightness and high efficiency are needed simultaneously. Further improvement in electroluminescence intensity (from the order of nanowatt (nW) to microwatt (μW) without changing the device geometry) also is desirable.